One type of offshore well equipment employs a floating platform, which may be used for drilling, production or both. One or more risers extend from wellhead equipment at the sea floor through openings in the platform. A riser tensioner applies tension to the riser. Wave and current can cause upward, downward and lateral movement of the platform relative to the riser.
The tensioner is made up multiple hydro-pneumatic cylinders spaced around the opening. If one of the cylinders fails, the upward forces exerted by the tensioner on the riser will no longer be balanced. The imbalance can cause bending of the upper portion of the riser, particularly where it passes through the opening. Even though a failed tensioner cylinder can usually be repaired without retrieving the riser, the bending may have damaged the riser. Retrieving a riser to replace a damaged upper portion of the riser may be very expensive.